Coatings

Introduction: There have been many changes in the coatings industry since my presentation in Boston in September, 1994. Most of these changes have resulted from Federal regulations and the technology blossoming as a result of dealing with the regulations.

1. Coating Manufacturers, Representatives

Ameron has purchased Devoe. Both very high quality manufacturers. Other manufacturers have also combined.

1.1. Importance of local sales/technical representative

Local rep is your first line of defense relative to technical questions, application conditions, dealing with factory leading to WARRANTY

Your evaluation of the local rep, friend or not, may, or may not, be the selection criterion for the successful supplier, but certainly is a valid reason for deciding which supplier you do not select

1.2. Beware the manufacturer that by-passes the local rep

Manufacturer that is willing to cut-out the local rep, does not have your interests at heart either

2. Coatings and Surface Preparation

2.1. Surface Preparation and Cleaning of Steel and Other Hard Materials by High and Ultrahigh-Pressure Water Jetting Prior to Recoating, SSPC-SP 12.

Replacing sand blasting in many cases

35-40,000 psi working pressure

Small water volumes (relatively), no major volumes of sand to collect/dispose of.

Minimal dusting

Removes chloride contamination

The National Shipbuilding Research Program, Surface Preparation & Coating Handbook, Carderock Division, Naval Surface Warfare Center, states that High Pressure Water Jetting (10-25 ksi) has far lower productivity rates (20-30 ft2/hr) than abrasive blasting. However all things change, and new pumps and nozzles achieve blast pressures of 35-40 ksi in the field have production rates of 80-100 ft2/hr, with abrasive blasting at 90-120 ft2/hr.

High pressure water jetting does not put a new surface profile on a surface. On an older, wasted surface, such as 30-40 years out of drydock, a new profile is needed and this will require an abrasive blast. Your friend, the Manufacturer's Rep is there to advise you.

2.2. Coatings Removal By Laser

Relatively new technology

Vaporizes coating by heating surface to about 250F

Vapors collected by vacuum enclosure, similar to vacu-blast

Skid or truck mounted for portability.

Units for sale only at this time: $200,000 to $3,000,000 each-too expensive for HNSA ships

Used mainly on high value operating aircraft. Eventually might be useful in ordnance or electronics antenna preservation, when and if available for specific task rental.

2.3. Surface Preparation

Still no free lunch, but some newer "snake oils" work much better than the "snake oils" of old.

Phosphoric acid, molybdates acted to convert rust to a moderately tightly adhering base coating that could be overcoated

Epoxy coatings normally applied at SOF temperature, or higher, with the surface temperature at least SF above the dew point.

Some newer coatings will go as low as 20F, and perhaps as low as OF.

These low temperatures are achieved by altering the catalysts and the resulting chemical reactions. If low temperature coatings are required, be very careful this is a normal product for the manufacturer, that he knows what the long term effects are on coating durability, and that he is willing to guarantee coating performance.

Kennedy used a low temperature cure International (Courtalds) epoxy primer in 1986-7. Except for local ice damage, it seems to be holding up well in 1996.

Epoxy and polyurethane (both catalyzed two component coatings) continue to cure and get harder as they age. This results in an overcoating "time window" in which another coating can be applied and stick/bond to the undercoat. If the "window" is exceeded, the undercoat will have to be roughened so the new coat will adhere. There is no fudging on this one. The window varies from product to product and is affected by the ambient temperatures between coats.

Some coatings claim a greater resistance to water penetration, etc by the addition of aluminum, stainless steel, glass, ceramic, or micaceous iron oxide flakes/platelates depending on the service or use intended.

They work.

2.5 Coatings

Labor for surface preparation, coating application, and clean-up is the most expensive part of painting whether you hire an outside contractor or use your own staff, that you already have on pay-roll.

Current costs and cash flow often dictate the paint that is purchased.

Buying Mil-Spec paint should be good enough. It is cheap and I can get a good deal from the supplier. It often isn't. Navy coatings are good, but usually not the high end in performance. Often a manufacturer has a higher performance version of the same type coating that will help maximize the impact of your labor dollar.

What are representative costs of some high performance coatings. Cost ranges are first given per gallon in small orders (less than 500 gallons), and then per gallon in 500 gallon orders.

o-20-30% Silicone Alkyd

$34.00

$20-22

o-30% Silicone Alkyd

$40.00

$25-30

o-Epoxy Mastics

$35-40

$20-25

An ideal topside system would include extensive surface preparation, epoxy mastic primer/encapsulants, and acrylic polyurethane topcoat. Systems similar to this are in use with an expected service life of about 15 years.

Problems: Polyurethane is a two part catalyzed system, requiring mixing and, perhaps losing, large amounts of product for even relatively small touch-ups. (It comes packaged that way) The polyurethane system is very hard and will require relatively extensive surface preparation to overcoat, i.e., blast cleaning.

A more user-friendly system starts off with surface preparation, an epoxy mastic primer/ encapsulant, and then a high quality 30% Silicone Alkyd. This system should have an expected service life of 10-12 years.

The Silicone Alkyd is a single component coating that does not require extensive surface preparation to overcoat. It is more familiar to a user. A container can be opened, a portion used, and the container sealed until the next use.

Massachusetts used the older Silicone Alkyd system in 1986 on the superstructure, and did not require a full overcoat, now in progress, until 1995. Spot touch-up had been on-going for several years.

Excessive paint film build-up is always a problem and will eventually fail in adhesion, either to itself, or the substrate. Massachusetts has build-ups of 60-100 mils, and is not unusual among HNSA ships. What is unusual, in 1989 Texas' exterior was blast cleaned to bare metal from keel to mast head being recoated with an epoxy/silicone alkyd system.

Natural oil-based or synthetic resin based (alkyd) were among the original decorative/protective coatings. Both have moderate water vapor transmission rates (good on wood, bad on steel) and require an inhibitive pigment for steel. Drying (film formation) is by reaction with oxygen.

Lead was discovered to be a dryer when added to the mixture. Later other metals were also found to be dryers: cobalt, zirconium and manganese.

Alkyds tend to absorb water and swell. They loose adhesion when immersed in water but usually recover when the substrate drys out. Repeated wet/dry cycling will cause coating disbondment.

The chemical reaction with oxygen continues from application until the coating becomes brittle and deteriorates. In sunlight, eventually the surface will chalk, decomposition products wash off, the binder become more brittle and decompose.

However, many alkyds need the presence of ultraviolet light to preserve the protection qualities. Coatings with high linseed oil content will yellow considerably indoors. Remember mixing blue with white when painting interior compartments?

Smelly paints: probably "short oil", with aromatic solvents. They dry faster, have greater gloss, are more brittle, do not wet the surface as well, and do not weather as well as "long oil" alkyds.

There are various alkyd resin modifiers: phenolics, epoxies, urethanes, and silicones. The silicone alkyds are of particular interest because of their gloss retention, weathering and ultra-violet resistance.

These coating are hybrids, combining technologies from other chemical categories. Generalizing, the hybrid coating is better than the straight oil-based, but not as good as a coating cured by chemical reaction.

Lead and chromates have severe toxicity problems as inhibitive pigments. There are many alternative inhibitors, used singly and in combination, all with specific strengths and weaknesses.

Water based alkyds do not in general meet the performance of solvent based alkyds.

Vinyls and Chlorinated Rubbers

Use of both systems started in the 1940s, chlorinated rubber being more popular in Europe than the U.S.

These coatings have very low moisture and oxygen transmission properties.

Vinyls and chlorinated rubbers are lacquers and are vulnerable to their solvents. This weakness helps in overcoating as the next coat solvent bonds (welds) with the surface of the earlier coat. Chlorinated rubber is attacked by almost all common solvents.

Adhesion of vinyls to steel is high over a coat wash coat primer (Navy Formula 117), a vinyl butyral primer.

Vinyl bottom paint systems have been used by the Navy since about 1960, usually over a near white metal blast (SSPC-10), and a coat of vinyl butyral wash coat primer. A great system except in fresh water (Philadelphia), because of soluble chromates. Cassin Young (DD-796) was inactivated at Norfolk in 1960 using the vinyl system, and stored in Philadelphia. Drydocking in Boston in 1981, I can testify the system was still in remarkably good condition.

Vinyl film toughness allows it to resist ice damage.

Very good coatings in the right place, vinyls and chlorinated rubbers are probably doomed because of the VOCs in their solvents.

Coal Tar and Coal Tar Epoxies

Coal tars go back to around 1000 A.D.

Used because of very low water penetration and stand up very well against disintegration in water. Also, relatively inexpensive.

Coal tars do not like sunlight;

Great color selection in black, or almost black.

Because of aromatic compounds, thought to be carcinogenic, use of coal tar epoxies is restricted by some countries. Although my reference notes no legislation to eliminate coal tar epoxies use in the U.S., as of 1996, great precautions are recommended both in application and removal due to flammability, toxicity, and carcinogenicity.

Coal tar epoxies are heavy and usually need at least two coats to assure coverage of discontinuities. However, the overcoat time window is very limited before the coating becomes too hard and too slick for the next coat to adhere.

The difference between the organic and inorganic coatings is the binder, often epoxy for the organic, and a silicate for the inorganic.

Performance differences: In a sea coast marine environment, with commercial blast cleaning, SSPC-SP 6, the service life estimate for an inorganic zinc with high-build epoxy system versus organic zinc with high-build epoxy system is 15 years versus 13.5 years.

However, the inorganic system requires more application expertise than the organic, both in regard to carefully monitoring ambient atmospheric conditions to assure appropriate inorganic zinc curing before topcoating, and in applying the proper dry film thicknesses of both the inorganic zinc and the topcoats.

Organic zincs are easier to top coat and are compatible with more coatings due to their denser surface. Based on average surface preparation for both types of zincs, organic zincs may outperform the inorganic product.

Inorganics are highly resistant to abrasion, sunlight, and solvents. On inactive ships, the Navy recommends no overcoating, but use the gray and not the reddish color!

Zinc coatings are not recommended on the underwater body, particularly old steel. In 1988, both Devoe and Ameron strongly recommended against using zinc silicates (inorganic) on decks, because point loads from walking, loading stores. etc would cause the coating to shatter, notwithstanding outstanding abrasion resistance.

Epoxies

We are talking of two-component catalyzed epoxy resins. These break into the polyamine hardeners and polyamide hardeners.

Polyamine hardeners are resistant to chemicals and solvents and are often used for lining tanks.

Polyamide hardeners are the most popular coatings on structural steel, with superior flexibility and durability. The standard Navy hull coating is polyamide chemistry (MIL-P-24441), the well-known 150 series formulas.

Epoxies have poor resistance to sunlight, as evidenced by chalking. Chalking in itself does not effect the initial corrosion performance. As chalking washes off the coating becomes thinner, and then performance is affected.

In maintenance areas, high solid, high build epoxies with good surface wetting characteristics have been developed to overcoat aged coatings, such as lead-containing alkyds. These are the "Mastics".

Applied to a marginally adhering base coating, they may well cause delamination.

These mastics are referred to as "surface tolerant", but there are questions as to how much contamination they will tolerate before performance is sacrificed.

Aluminum flake filled mastics are being used to successfully overcoat old alkyd lead containing coatings. The mastics are then themselves overcoated. This delays deleading.

As noted earlier, epoxies have a definite overcoat time window, not too early and not too late.

Also as noted, application temperatures/humidity are critical. 50 F is normally the lower end. Devoe has a regular product, No. 235, that will cure at 0 F, a derivative of No. 230 with more usual curing characteristics. A product of this type allows potential drydocking and painting in Boston during December into March.

Epoxies can be strong skin sensitizers and respiratory irritants and have to be handled very carefully. Thermal decomposition products are also of concern during structural maintenance work.

Some epoxies can, and do, cure underwater. My experience has been varied: temperature appears important as are considerations involving the effects of cathodic protection on the area to be plugged/coated. Leaking fuel is also a real problem.

[Webmasters Note - The Department of Defense (DoD) Corrosion Exchange e-portal is life-cycle oriented and serves as a leader in corrosion research, industry coordination, and information dissemination to all individuals and organizations in the DoD, academia, and industry who have an interest in corrosion-particularly as it relates to DoD weapon systems, equipment, related commercial assets, and infrastructure. The DoD Corrosion Exchange (DoDCE) provides collaboration and information sharing services and tools allowing members to collect, analyze, synthesize, and distribute scientific and technical corrosion-related information throughout the Corrosion Prevention and Control (CPC) community.
Membership to the Department of Defense Corrosion Exchange is free of charge.]